A storage system typically comprises one or more storage devices into which information may be entered, and from which information may be obtained, as desired. The storage system includes a storage operating system that functionally organizes the system by, inter alia, invoking storage operations in support of a storage service implemented by the system. The storage system may be implemented in accordance with a variety of storage architectures including, but not limited to, a network-attached storage environment, a storage area network and a disk assembly directly attached to a client or host computer. The storage devices are typically disk drives organized as a disk array, wherein the term “disk” commonly describes a self-contained rotating magnetic media storage device. The term disk in this context is synonymous with hard disk drive (HDD) or direct access storage device (DASD).
The storage operating system of the storage system may implement a high-level module, such as a file system, to logically organize the information stored on volumes as a hierarchical structure of data containers, such as files and logical units. For example, each “on-disk” file may be implemented as set of data structures, i.e., disk blocks, configured to store information, such as the actual data for the file. These data blocks are organized within a volume block number (vbn) space that is maintained by the file system. The file system may also assign each data block in the file a corresponding “file offset” or file block number (fbn). The file system typically assigns sequences of fbns on a per-file basis, whereas vbns are assigned over a larger volume address space. The file system organizes the data blocks within the vbn space as a “logical volume”; each logical volume may be, although is not necessarily, associated with its own file system.
A known type of file system is a write-anywhere file system that does not over-write data on disks. If a data block is retrieved (read) from disk into a memory of the storage system and “dirtied” (i.e., updated or modified) with new data, the data block is thereafter stored (written) to a new location on disk to optimize write performance. A write-anywhere file system may initially assume an optimal layout such that the data is substantially contiguously arranged on disks. An example of a write-anywhere file system that is configured to operate on a storage system is the Write Anywhere File Layout to (WAFL®) file system available from Network Appliance, Inc., Sunnyvale, Calif.
A plurality of storage systems may be interconnected to provide a storage system environment configured to service many clients. Each storage system may be configured to service one or more volumes, wherein each volume stores one or more data containers. Yet often a large number of data access requests issued by the clients may be directed to a small number of data containers serviced by a particular storage system of the environment. A solution to such a problem is to distribute the volumes serviced by the particular storage system among all of the storage systems of the environment. This, in turn, distributes the data access requests, along with the processing resources needed to service such requests, among all of the storage systems, thereby reducing the individual processing load on each storage system. However, a noted disadvantage arises when only a single data container, such as a file, is heavily accessed by clients of the storage system environment. As a result, the storage system attempting to service the requests directed to that file may exceed its processing resources and become overburdened, with a concomitant degradation of speed and performance.
One technique for overcoming the disadvantages of having a single file that is heavily utilized is to stripe the file across a plurality of volumes configured as a striped volume set (SVS), where each volume is serviced by a different storage system, thereby distributing the load for the single file among a plurality of storage systems interconnected as a cluster. A technique for data container (file) striping is described in U.S. patent application Ser. No. 11/119,278 of Kazar et al., entitled STORAGE SYSTEM ARCHITECTURE FOR STRIPING DATA CONTAINER CONTENT ACROSS VOLUMES OF A CLUSTER. File striping improves raw performance and reliability across the cluster of storage systems by distributing data among different storage systems generally based on an offset within the file at which the data is located. Each storage system is configured to serve an underlying physical volume embodied as an aggregate comprising one or more groups of disks.
It is generally desirable to eliminate duplicate data on storage resources, such as disks, and to ensure the storage of only a single instance of data to thereby achieve storm age compression. Such elimination of data duplication (de-duplication) also results in a more efficient use of cache memory. De-duplication in the exemplary file system ensures that if two blocks contain the same data, and thus have the same content, only one copy of the block is maintained on disk and two references (pointers) are directed to (i.e., share) that block. Low-level indirect blocks typically include vbn pointers to data blocks, which vbns are mapped to disk block numbers (dbns) on disk. In the case of de-duplication, different vbns refer to the same dbn. For example, if data stored at vbn 15 and vbn 32 were identical, that data would be retrieved from disk from the same location and, therefore, cached at the same location in memory. This enables not only a reduction of storage space consumption, but also a performance improvement since a data block that is shared by many files is more likely to be resident in cache when needed again.
An example of a technique for eliminating duplicate data is described in U.S. patent application Ser. No. 11/105,895, filed on Apr. 13, 2005, entitled METHOD AND APPARATUS FOR IDENTIFYING AND ELIMINATING DUPLICATE DATA BLOCKS AND SHARING DATA BLOCKS IN A STORAGE SYSTEM, by Ling Zheng, et al, the contents of which are hereby incorporated by reference. Here, data de-duplication operations are performed on fixed size blocks. When a new block is to be stored, a hash value is computed as a fingerprint of the block. The fingerprint is then compared with a hash table containing fingerprints of previously stored blocks. If the new block's fingerprint is identical to that of a previously stored block, there is a high degree of probability that the new block is identical to the previously stored block. In such a case, the two blocks are compared to test whether they are indeed identical. If so, the new block is replaced with a pointer to the previously stored block, thereby reducing storage resource consumption.
However, de-duplication generally takes place at the aggregate level; that is, for sharing to occur, all references to that data must occur within the same aggregate. As noted, file striping across a storage system cluster typically distributes data among the different storage systems based on an offset within a file, not based on the content of the data. As a result, a data access request directed to a first block of a first file may be forwarded to a first storage system, whereas an access request to a second block of that file may be forwarded to a second storage system. Thus, despite the content of the data, the access requests are directed to different systems. Accordingly, the efficiency of de-duplication is compromised because only a portion of the data is available to search for a duplicate; there is no attempt to route identical blocks of data onto a single storage system. The present invention is directed to ensuring that a block of data is stored on a stores age system based on the content of that data block rather than based on its offset within a file.